Question about the MK12 rifle used by the Seal Teams

I've been reading a book ("Lone Survivor") by Marcus Luttrell about the battle battle in Afghanistan that took the lives of his buddies. It's a great read (although very sad) and it's not geared about the weapon systems in detail but he mentions his "MK 12" rifle in a number of places. I've done some basic googling on it and it appears to be a hot-rodded AR. The question I haven't been able to answer is what type of gas system it has on it. Does anyone have any info on that? (is it a piston system or just an AR with a non-standard barrel length and free float handguard?)

Anyway that's my question. If anyone's interested in our Seal teams the book is a good read. I know that even in my prime I couldn't have made the first day of their training I don't think. :)

Have a good one,
Dave

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